In ATM switching system, VP/VC switching between a plurality of physical ports is usually required, with one typical example being DSLAM (Digital Subscriber Line Access Multiplexer). Generally speaking, one DSLAM device needs to provide several network side interfaces and several subscriber ports. But many ATM switching chips provide only one UTOPIA (L2) interface, which is apparently unable to satisfy the requirements in most cases, for example, small size DSLAM usually demands subscriber interfaces over 32. Therefore, the UTOPIA address extension for ATM switching chips is inevitable. Some chips extend address by the way of multiple multiplexing through self-defined interface such as Any-PHY interface. However, this way has its own limitation—both the ATM switching chip and the physical layer chip must accept the self-defined private interface, setting restriction on the system flexibility.
In the PHY address extension technique, there must be an interface to identify the different ports after extension. The general method is to add port identifier at the ATM cell head. But many ATM switching chips only provide interfaces for users modifying VPI/VCI (some chips provide private interface in user defined range H5/UDF). Therefore, adding port identifier in VPI/VCI can be considered as a universal method (“universal” here means that this method is not limited to the chip types). But, the above method also has the following defects: the space of the VP/VC table in the switching chips is limited, for example, if the space of the VC table for the use of the port needing extension is 4K, and this port is to be extended to 32 ports, then 5 bit is needed to identify the ports, then the valid VPI/VCI can only be 7 bit, thus the valid VPI/VCI value is limited. In DSLAM system, the demand on the connection number that each port on the subscriber side can support is not high (for example, each MODEM supports 8 connections), but the VPI/VCI value of these connection is not restricted (VPI: 0˜255, VCI: 0˜65535), so 7 bit VPI+VCI is evidently insufficient. If extension for more ports has to be supported, for example, the large sized DSLAM demands subscriber ports over 1 k, it is even more insufficient.